Magnetic recording apparatuses for magnetically recording and reproducing information keep advancing as large-capacity, high-speed, inexpensive information storage means. In particular, the recent progress of hard disk drives (HDDs) is remarkable. The technique of further increasing the density of the HDD has advanced as a compilation of a plurality of component technologies such as signal processing, mechanical servo, heads, media, and HDI. Recently, however, the thermal stability problem of a medium has become serious as a factor that prevents the development of high-density HDDs.
In magnetic recording using a multi-grain medium formed by a conventional polycrystalline magnetic grain thin film, there is a tradeoff relationship between low noise, a high thermal stability, and a high writability. This is an essential problem that determines the limit of the recording density.
If Ku of a medium magnetic film is increased in order to achieve both a small grain size and high thermal stability, a recording coercive force Hc0 of the medium, i.e., the coercive force when a magnetic head performs high-speed magnetization reversal rises, and a magnetic field necessary for saturation recording increases in proportion to Hc0.
The recording magnetic field generated from the recording head and applied to the medium depends on, for example, the recording magnetic pole material, magnetic pole shape, spacing, medium type, and film thickness, in addition to an electric current to be supplied to a recording coil. Since, however, the size of the distal end portion of the recording magnetic pole decreases as the density increases, the magnitude of the generated magnetic field is limited, and at most about 15 kOe.
Recently, the increase in Hc0 and the decrease in head magnetic field are making recording difficult, and an overwrite OW used as an index of the degree of recording, i.e., a value indicating the degree of erasure of previous magnetization information by overwriting is becoming worse. In particular, even within the range of 5° C. to 55° C. as the operating temperature of the HOD, Hc0 increases as the temperature decreases. Therefore, OW worsens as the temperature decreases.
The existing CoCrPt-based magnetic recording layer shows a very high SNR, so it is extremely difficult to replace this material with another material. It is still possible to increase Ku and Hco of this magnetic recording layer by, for example, changing the composition, and recording is possible near room temperature unlike when using an FePt ordered alloy.
When locally heating the medium as described above by using some means, OW can be improved by decreasing Hc0 of the heated portion.
One method is the thermally assisted magnetic recording method.
In the thermally assisted magnetic recording method using a multi-grain medium, it is possible, by using fine magnetic grains that sufficiently reduce noise, to use a recording layer having a high Ku near room temperature in order to secure a high thermal stability. A medium having a high Ku like this is incapable of recording near room temperature because the magnetic field necessary for recording becomes larger than the generated magnetic field of a recording head. In the thermally assisted magnetic recording method, however, a heating means using an optical beam or the like is installed near the recording magnetic pole, and recording is performed by locally heating a medium such that Hc0 of the heated portion becomes lower than the recording magnetic field from the head.
Thermally assisted magnetic recording has the following problems.
First, it is difficult to decrease the beam diameter to about 100 nm in order to perform local heating at about 300° C. If the beam diameter is not decreased, the efficiency decreases, so a higher power is necessary.
To increase the recording density in the line direction, the temperature gradient must be increased, but this is also difficult.
If the temperature gradient is not increased in the track direction, information on an adjacent track is easily lost.
It is necessary to align the positions of the temperature gradient and magnetic field gradient by positioning the heating source and recording magnetic pole as close as possible. However, this is also very difficult to achieve.
This alignment becomes more difficult as the bit size and beam size decrease, and when skew is taken into consideration.
Examples of assumed media are FePt and CoPt ordered alloys and Co/Pt and Co/Pd multilayers each having a high Ku. When using an unprocessed continuous medium such as a bit patterned medium, however, it is difficult to secure a high SNRm like that of the existing CoCrPt alloy-based granular medium.
A method of forming a heat sink for cooling without diffusing the heat on the medium side in order to increase the temperature gradient as described above has been proposed. However, the grain size and crystal alignment of the magnetic recording layer decrease if it becomes impossible to form a thick nonmagnetic interlayer that is indispensable in the conventional polycrystalline perpendicular medium, and a soft magnetic underlayer is practically omitted. This makes it impossible to reflux the magnetic flux of a head, and decreases the magnetic field gradient.
Since heating to about 300° C. modifies or evaporates a lubricant, it is also necessary to change the lubricant.
Although it may not be necessary to raise the temperature as high as the Curie point, the above-mentioned problems remain almost unchanged when heating is performed at 200° C. or more.